A typical insulated electric power cable generally comprises a conductor in a cable conductive core that is surrounded by several layers of polymeric materials including an inner semiconducting shield layer (conductor or strand shield), an insulating layer, an outer semiconducting shield layer (insulation shield), a metallic wire or tape shield used as the ground phase, and a protective jacket. Additional layers within this construction such as moisture impervious materials are often incorporated. The present invention pertains to the inner semiconducting shield layer, i.e., the conductor shield.
Semiconductive shields have been used in power cables as shields for the cable conductor and insulation for many years. The conductor shield is typically extruded over the cable conductor to provide a layer of intermediate conductivity between the conductor and cable insulation in the power cable. Conventional compositions for these conductor shields include a base polymer as the predominant component of the composition compounded with carbon black to provide conductivity for the composition and may include various additives.
The invention relates to semiconductive shields for use in electrical conductors such as power cables, and particularly to a vulcanizable semiconductive conductor or bonded insulation shield composition that exhibits improved physical properties and processability compared to known semiconductive conductor and bonded insulation shields.
The semiconductive shield used to screen the electrical conductor is conventionally formed by dispersing various furnace-type carbon blacks, such as ASTM N-472 or Cabot XC72® type grade carbon blacks, in an ethylene copolymer resin base. These furnace blacks often have poor dispersion characteristics in polymers and contribute high levels of ionic contaminants. Consequently, protrusions and contaminants occur at the cable's shield/dielectric interface, causing increased stress gradients in an electrical field. This electrical field enhancement, combined with the migration of water and ions into the insulation, may lead to the formation of water trees and subsequent dielectric breakdown and premature cable failure.
Other commercially available high performance semiconductive shield compositions contain other types of carbon blacks, such as acetylene blacks, and an ethylene/ethylacrylate copolymer, ethylene/vinylacetate copolymer, ethylene/butylacrylate copolymer or blends of these materials with polyethylene. These materials typically contain reduced levels of ionic contamination and exhibit good dispersion and very smooth extrusion surfaces. Such shield compositions have a high viscosity due to the high carbon black loadings needed to achieve adequate conductivities and, therefore, abrade and/or corrode cable extrusion equipment. This wear results in poor extrusion cable surfaces and interfaces, thus reducing the shield's electrical performance properties.
Efforts have been made to improve semiconductive shield compositions. High performance semiconductive conductor shield compositions that include an ethylene/vinyl acetate copolymer, acetylene carbon black, and an organic peroxide cross linking agent are often used for these applications. Vinyl acetate resins, however, may only be used with aluminum conductors because they are corrosive to copper conductors. Furthermore, high loadings of acetylene black combined with ethylene/vinyl acetate resin lead to the formation of acids in the extruder which then corrode and abrade extrusion die tooling, resulting in cable dimension variations over time.
The primary purpose of the semiconducting conductor shield between the conductor and insulation in an electrical power cable is to ensure the long term viability of the primary insulation. There is always a need for improved semiconductive conductor shield compositions that balance cost and performance.
U.S. Pat. No. 6,086,792 to Reid et al. discloses a semiconducting composition comprising an olefinic polymer and a carbon black with a particle size of at least 29 nm.
International Application WO 01/40384 to Achetee et al. discloses carbon blacks and semiconducting compositions where the carbon black has a particle size 22-39 nm, an Iodine Number from about 64 to about 120 mg/g and a tinting strength of about 90% or less.
U.S. Pat. No. 5,877,250 to Sant discloses carbon black and polymers containing carbon black, wherein the carbon black has a particle size not greater than 20 nm and an Iodine Number of 64-112 mg/g. It is disclosed that improved processability is imparted by the use of the particular carbon black, although the use of such a carbon black to manufacture a semiconductive composition is not disclosed.
U.S. Pat. No. 5,556,697 to Flenniken (Flenniken '697) discloses a Vulcanizable semiconductive shield compositions contain a linear, single-site catalyzed polymer formed by polymerizing ethylene with at least one comonomer selected from C3 to C20 alpha-olefins; a carbon black selected from furnace carbon blacks that contain ash and sulfur in amounts of 50 ppm or less. Flenniken '697 further discloses adding a ethylene vinyl acetate silane terpolymer. This has the disadvantage of reacting and cross linking with certain carbon blacks and over time in the presence of moisture. Because of this, the compound may be soft and prone to mar and scratch in the cable making equipment. A further disadvantage is that the conductor is not able to be preheated to a high temperature.
U.S. Pat. No. 6,864,429 to Easter discloses a semiconducting shield composition having enhanced electrical aging performance as measured by the accelerated water treeing test (AWTT) and the accelerated cable life test (ACLT). Carbon blacks used in the present invention have a particle size from about 15 to about 22 nanometers, preferably from about 18 nm to about 21 nm (as measured by ASTM D3849-89), an Iodine number from about 115 mg/g to about 200 mg/g, preferably from about 120 mg/g to about 150 mg/g (as measured by ASTM D 1510) and a DBP oil absorption of from about 90 cm3/100 g to about 170 cm3/100 g, preferably from about 110 cm3/100 g to about 150 cm3/100 g (ASTM D2414). N110 falls in this range. However, Easter doesn't disclose the effect of the polymer matrix.
U.S. Pat. No. 5,889,117 to Flenniken discloses a semi-conductive or insulating composition including an ethylene/octene copolymer and at least one additional polymer, such as ethylene/vinyl acetate. The composition may also include carbon black and other additives. The composition may be used as a semi-conductive or insulating layer in applications such as electrical cables. A further advantage of the polymeric formulations claimed is that they blend well and exhibit lower adhesion to crosslinked polyethylene, thus providing increased and continued strippability of the resultant products. This decreased adhesion is preferable, for example, because it increases the strippability of the polymeric composition from other compositions to which it is adhered. For example, decreased adhesion in the case of electrical cable allows for easier strippability of the semi-conductive shield from an underlying insulating material, with concurrently decreased pick-off, i.e., decreased amounts of polymer material residue left on underlying layers. Flenniken '117 doesn't disclose improved AWTT performance. Ethylene/vinyl aceatate is also and expensive polymer costing the same or more than ethylene/octene copolymer
International Application WO/2007/092454 to Kjellqvist, et al. discloses a polymer composite made from (i) a phase I material consisting essentially of a polar copolymer of ethylene and an unsaturated ester having 4 to 20 carbon atoms; (ii) a phase II material consisting essentially of a nonpolar, low density polyethylene; and (iii) a conducting filler material dispersed in the phase I material and/or the phase II material in an amount sufficient to be equal to or greater than the amount required to generate a continuous conducting network in the phase I and phase II materials. The invention also includes articles made from the polymer composite. This has the disadvantage of having a polymer copolymer and of having to control the dispersion of the phases to have a sufficiently fine conductive network.
Additional examples of polymer compositions used as shields in power cables are found in the disclosures of U.S. Pat. Nos. 4,612,139 and 4,305,846 to Kawasaki et al.; U.S. Pat. No. 6,455,771 to Han et al.; U.S. Pat. No. 4,857,232 to Burns, Jr.; and U.S. Pat. No. 3,849,333 to Lloyd et al.
It would be desirable to have a conductor shield material with improved performance that does not require the use of expensive conductive carbon blacks, is mar resistant, can survive conductor preheating, and uses a lower cost blend of polymers as performance must always be balanced with cost in the manufacture of electric cable.